The present invention relates to a method for generating a sequence of spin echo signals in NMR spectroscopy, in which a sample placed in a homogeneous magnetic field is excited by a sequence of rf pulses resulting in the generation of spin echo signals.
There have been known several methods for generating spin echo signals. However, it is common to all these methods, that they include measures intended to reverse the induction decay occurring as a result of dephasings of the excited spins, due to inhomogeneities of the magnetic field, so as to achieve a new signal by corresponding rephasing. A typical example to be mentioned in this connection is the Carr-Purcell pulse sequence where a 90.degree. excitation pulse is followed by a number of 180.degree. pulses which have a rephasing effect and the result that a spin echo signal occurs after every 180.degree. pulse. By adding up the spin echo signals, it is now possible to achieve a considerable improvement of the signal-to-noise ratio. A similar effect can be achieved also by switching gradient fields between the spin echo signals. It has also been known to make use of such methods for generating a sequence of spin echo signals in nuclear spin tomography, and to give the individual spin echo signals different codings so that in the extreme case a single sequence of spin echo signals will be sufficient to obtain the signals required for producing a two-dimensional image.
However, the known methods for generating a sequence of spin echo signals are connected with the drawback that they are still very time-consuming because the occurrence of the individual spin echo signals is separated by the switching processes necessary for the interposition of the gradient fields and because these processes require a finite period of time. Although it would be possible, technically, to switch gradients so rapidly that the method known from DE-A 34 34 161 would permit the generation of an image in a few milliseconds, such measuring sequences are suited only for the generation of a sequence of only a few images as it is an inherent feature of such sequences that they result in extreme mechanical stresses for the measuring system and-- as a result thereof--in an extreme noise level which would be unacceptable for the patient. Switching gradients at such extreme speeds would in addition lead to conditions where the limit values admissible for examinations of patients would be exceeded by far.
Consequently, it is not possible with the aid of these techniques to generate a continuous sequence of cross-sectional images--similar to the ultrasound process, for example.